Digital reduction of the Peak to Average Power Ratio (PAPR) of a complex radio signal, while filtering the signal to remove out of band emissions, provides a way to reduce the peak demand on a power amplifier. This enables improved efficiency and cost of a radio system with a small but acceptable degradation in signal quality or Error Vector Magnitude (EVM).
Orthogonal frequency division multiplexing (OFDM) technologies have complex radio signals with many closely spaced sub-carriers each of which can have a very different EVM requirement. However the close frequency spacing makes it more difficult to provide differentiation from an EVM perspective while reducing the PAPR. Existing technologies do not efficiently differentiate between sub-carriers and also require reserved sub-carriers. In addition, time domain technologies such as global system for mobile communications (GSM), wideband code division multiple access (WCDMA) and code division multiple access (CDMA) need to be handled together with OFDM technologies in the same power amplifiers.
With the use of OFDM technologies, the signal is composed of an array of sub-carriers of varying tolerance to degradation in signal quality. Existing time domain solutions are unable to make a distinction between the sub-carriers, and so are limited in the amount of peak power reduction (PPR) that can be applied by the least tolerant sub-carrier. Some existing solutions do not differentiate between traffic sub-carriers and require reserved tones or reserved sub-carriers that take away from the allocated spectrum for normal communication e.g., APPARATUSES AND A METHOD FOR REDUCING PEAK POWER IN A TRANSMITTER OF TELECOMMUNICATIONS SYSTEMS, U.S. Patent Application Publication No. 2009/0176466 Al, Jul. 9, 2009; and SYSTEM AND METHOD FOR REDUCING PEAK TO AVERAGE POWER RATIO IN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING SIGNALS USING RESERVED SPECTRUM, U.S. Pat. No. 7,583,583, Filed Dec. 15, 2005. Other existing solutions differentiate between traffic sub-carriers but still require reserved sub-carriers that take away from the allocated spectrum for normal communication. e.g., METHOD AND SYSTEM FOR ADAPTIVE PEAK TO AVERAGE POWER RATIO REDUCTION IN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING COMMUNICATION NETWORKS, U.S. Patent Application Publication No. 2009/0092195 A1, Apr. 9, 2009.
When OFDM and time domain technologies are used in the same band (sharing the same transmitter) then the PPR method used may be a time domain method having the above mentioned limitations.